Mass filter with sheet electrodes on each side of the analyzer rod that intersect on the ion beam axis



3,280,326 F THE 1966 KARL-GEORG GUNTHER MASS FILTER WITH SHEET ELECTRODES ON EACH SIDE O ANALYZER ROD THAT INTERSECT ON THE ION BEAM AXIS Filed May 1, 1964 United States Patent 9 Claims. or. 250-419 My invention relates to mass filters for separating ions of different specific electric charges by shooting them toward a collector electrode through an electric high-frequency field between electrodes which are elongated parallel to the axis of the trajectory path, so that only ion-s of certain specific charges or mass can pass through the field to the collector electrode, whereas other ions are deflected laterally onto the field-producing analyzer electrodes. Mass filters operating on this separation principle are known from U.S. Patent-s No. 2,939,952, No. 2,950,389, No. 3,075,076.

A four-pole (quadrupole) mass spectrometer (filter) according to Patent No. 2,939,952, having four elongated rod electrodes for producing the analyzer field, affords a mass separation or resolving power up to a given optimum determined by the geometry of each particular quadrupole apparatus. A mass filter operating on the same principle but with only one analyzer rod electrode (application of von Zahn, Serial No. 242,224, filed December 4, 1962 and issued July 27, 1965 as United States Patent No. 3,197,633) employs, in eifect, a one-quarter sector of the above-mentioned quadrupole analyzer, its single rod electrode being located opposite two sheet metal members which form an angular structure and extend in perpendicular relation to each other, the planes of the two sheets being angularly displaced 45 relative to the coordinate planes defined by the respective reference axes X and Y of the mass filter. This affords improving the resolving power for a given axial length of the analyzing field. However, such a monopole apparatus is asymmetrical, particularly with respect to the locality where the ions are shot into the analyzing system. As a result, the available space is unfavorably utilized when the analyzer system is built into a conventional spectrometer tube, unless resort is taken to giving the ion source itself an asymmetrical design, an expedient likewise tending to cause difiiculties. Furthermore, at dimensions comparable with other mass filters of this general type, the area utilizable for the ion beam shot into the analyzing field is rather small. These various reasons conjointly lead to a comparatively low sensitivity of the monopole spectrometer.

It is an object of my invention to provide a mass filter generally of the above-mentioned type which combines the advantages of the quadrupole system with that of a monopole system but avoid-s the disadvantages heretofore encountered with each of them. More specifically, it is an object of the invention to devise a mass filter system which, compared with those mentioned above, exhibits an improved resolving power conjointly with increased sensitivity.

According to my invention, I provide a mas-s filter, designed and operating on the principle set forth above, with elongated analyzer electrode rods which extend between the ion source and the collector electrode means to produce an alternating electric field for causing ions of certain specific electric charges or masses to be laterally deflected onto the rods, rather than passing through the analyzer field onto the collector means; and I mount the just-mentioned analyzer rods in at least two symmetry planes that extend through the source-collector axis of the system, in this respect corresponding generally to the above-mentioned quadrupole systems. However, a mass filter system according to the invention is further provided with intermediate thin sheet electrodes that extend in the just-mentioned symmetry planes and are joined with each other on the above-mentioned axis of the system.

According to one of the more specific features of my invention, the symmetry planes, and consequently the thin sheet electrodes extending in these planes, form right angles with each other, and four analyzer rods electrodes are located in the resulting four special sectors respectively.

In a mass filter according to the invention, all of the analyzer rods can be impressed by the same field voltage:

wherein U denotes the magnitude of a direct-voltage component, and V is the amplitude of an alternating-voltage component.

A single collector electrode may be provided for all of the analyzer rod electrodes. However, according to another feature of our invention, a separate collector electrode is provided for each analyzer rod electrode and receives the ions that pass through the particular one seetor space of the analyzer field.

According to still further features of the invention, the analyzer rod electrodes may be given respectively difierent diameters. The spacing r of the respective analyzer rod electrodes from the axis of the system may be made different. Thus, for measuring different masses of given respective magnitudes, the distances of the respective analyzer rods from the axis may be equal to r =c-m wherein c is a constant and m denotes the particular mass that is to be separated onto the analyzer rod having the distance r from the system axis.

According to still another feature of our invention, the analyzezr rod electrodes, regardless of whether they have the same or respectively different diameters, are impressed by respectively different voltages, each corresponding to the composite form U+V- cos wt As mentioned, the intermediate sheet electrodes are thin-walled. Suitable, for example, are metal sheets of about 0.1 to 0.5 mm. thickness. Also applicable is wire mesh consisting of wires of about 0.3 mm. diameter, for example.

The invention will be described hereinafter with reference to the accompanying drawings, in which:

FIG. 1 shows schematically a cross section of a known quadrupole mass filter;

FIG. 2 is a schematic cross section of a monopole filter, and FIG. 2a shows in cross section one way of mounting such a filter into a tubular housing;

FIG. 3 is a cross section through an embodiment of a mass filter exemplifying the invention;

FIG. 4 is a cross section of a different embodiment of the invention;

FIG. 5 illustrates in cross section still another example of a mass filter according to the invention; and

FIG. 6 shows schematically a lateral view of a gasanalyzing device incorporating a mass filter according to FIG. 3.

The prior-art mass filter shown in FIG. 1 corresponds to the one known from U.S. Patent 2,939,952. Symmetrically mounted in a cylindrical envelope 1 are four analyzer rod electrodes 2, 3, 4 and 5, all of the same length. Each two diametrically opposite rods are connected to a voltage of the form U+V- cos wt 2 and +U+V- cos wt 2 wherein U and V have the above-defined meanings. These voltages are applied through insulated and sealed in-leads 6 and 7. The coordinate directions of the main planes of symmetry are indicated at X and Y.

FIG. 2 represents schematically the above-mentioned monopole filter. Located on both sides of the single analyzer rod 1d are metal sheets 11 and 12 which extend in respective planes X and Y parallel to the rod axis and at an angle of 90 to each other. The sheet planes are angularly displaced 45 counterclockwise from the main symmetry planes X and Y.

In FIG. 1, as well as in FIG. 2, a shaded circular area at 8 or 13 indicates the locality where the ions are shot into the analyzer system. It will be noted that the ionentering area 13 in FIG. 2 is eccentric to the intersection line of the two sheets 11 and 12.

FIG. 2a shows that, when the monopole system is built into an ordinary spectrometer tube, the space utilization is unfavorable and that the outlet opening of the ion source must be eccentric.

In the improved mass filter according to the invention, as exemplified by the embodiment shown in FIG. 3, four analyzer rods 31 to 34 are mounted in an envelope which is grounded. A voltage supply lead 35 connected to a ring lead 36 applies to all of the four rods the same type and magnitude of voltage U+V- cos wt Metallic sheet members 37 and 38 extend in the two symmetry planes and, in this embodiment, define 90 angles with each other. The area of the ion beam is indicated by shading at 39. The sheet electrodes 37 and 38 can be connected to any desired voltages with the aid of insulated and sealed in-leads 40. The main'planes of symmetry are denoted by X, Y and X, Y.

As shown in FIG. 6, the envelope 36 contains an ion source 61 at one side of the group of analyzer electrodes and is provided with collector electrode means 62 which in this particular embodiment consist of 4 separate collectors for the respective analyzer rods. The envelope 30, kept at vacuum pressure, is vacuum-tightly sealed and has a neck 30 for connection to a supply of gaseous mixture to be investigated. The rod electrodes are electrically connected to a voltage source 64 which supplies a highfrequency voltage and a superimposed direct voltage as explained above. The current due to the ions reaching the collector electrode means 62 is amplified by an amplifier 65 and supplied to a recorder 66 or other measuring instrument for indicating the mas spectrum of the mixture being analyzed.

With respect to theory, performance and selection of voltages, reference may be had, if desired, to the abovementioned patents, particularly Patent No. 2,939,952.

The filter apparatus according to the invention as described above with reference -to FIGS. 3 and 6 affords the following advantages. The apparatus is completely symmetrical. The entire shaded area 39 is available for the ions to, enter into the analyzer field, this area corresponding to approximately four times the comparable area in a mass spectrometer of the monopole type, thus securing a high sensitivity. The analyzer electrodes rods are all connected to the same voltage, thus permitting a simplified mechanical design as well as a simplified high-frequency voltage source. The resolving power corresponds to that of the monopole spectrometer although the above-mentioned disadvantages of the latter are avoided.

It is sometimes necessary or desirable to simultaneously measure or record two or more different kinds of gases by means of the same mass filter. An apparatus according to the invention readily permits such a performance. For this purpose, the four '(or more) analyzer electrode rods are given difierent respective diameters and difierent respective distances from the axis of the system. This can be done in such a manner that the same voltage U+V- cos wt 2 is applied to the respectively different electrode rods and, relating to an apparatus with four analyzer quadrants, four diiferent masses are filtered out in accordance with the equation m c/r wherein c is a constant. Accordingly, the mass filter must have a corresponding number of collector electrodes and output circuits which permit a separate detection or indication of the respectively different masses. 7

FIG. 4 shows an embodiment which, when operated in the just-mentioned manner, will permit a simultaneous spectrographic analysis of different masses. The envelope 41 comprises four analyzer rods 42, 43, 44, 45 which have respectively different diameter and difierent radial distances r from the axis of the system. These distances are chosen in accordance with the formula m =const/r wherein m denotes the particular mass to be separated by the rod electrode and r denotes its particular radial spacing from the system axis. A ring conductor 46 supplies all four analyzer rods with the same analyzer voltage -U+V- cos wt 2 The thin sheet electrodes 47 and 48 in this embodiment extend in the planes of symmetry and form angles with each other. The symmetrically located trajectory area for the ions is indicated by shading. Five sealed inleads for the supply of voltage are shown at 42a, 43a, 44a, 45a and 4?.

While in the embodiments so far described, four analyzer rod electrodes are provided, any desired larger number of such rods may be used, in which case the correspondingly mounted intermediate sheet electrodes, as a rule, form acute angles between each other.

A mass filter of the latter type is exemplified .by FIG. 5. The tubular envelope 51 encloses a total of eight analyzer rods 52 which, in this embodiment, have all the same diameter and are all located symmetrically to intermediate thin sheet electrodes 53. The trajectory area of the ions, indicated by shading, is likewise symmetrical to the system axis. Denoted by 54 and 55 are sealed inleads for impressing the necessary operating voltages upon the electrodes.

An embodiment with more than two pairs of analyzer rods may likewise be given rods of difierent diameter and difierent radical spacing from the symmetry axis, and/ or the angular ranges between the sheet electrodes may be non-uniform. Such and other modifications afiord further possibilities of improving the utilization of the available space or simultaneously analyzing a larger number of respectively different masses.

I claim:

1. An electric mass filter, comprising an evacuable envelope, an ion source, collector electrode m'eans spaced from and defining with said ion source an ion path axis in said envelope, pairs of elongated analyzer electrode rods extending between said source and said collector electrode means in axially spaced relation to both andradially spaced from said axis for applying an alternating electric field for lateral deflection of ions, said analyzer electrode rods being symmetrically positioned to at least two symmetry planes through said axis, intermediate sheet electrodes intermediate two of said symmetry planes, said sheet electrodes having a substantially uniform thickness smaller than that of said analyzer electrode rods and extending radially through and from said axis and between said analyzer electrode rods, said ion source being mounted on said axis, said sheet electrodes being at least as long as said rods in axial length, and voltage supply means connected to said analyzer electrode rods for supplying determined voltages to said analyzer electrode rods.

2. An electric mass filter according to claim 1, wherein said two of said symmetry planes and said sheet electrodes intersecting each other at right angles.

3. An electric mass filter according to claim 1, wherein said two of said symmetry planes and said sheet electrodes intersecting each other at right angles, and said analyzer electrode rods being four in number and extending in the resmctive four quadrants formed by another two of said symmetry planes and separated by said sheet electrodes.

4. An electric mass filter according to claim 1, wherein said voltage supply means provides the same voltage to each of said analyzer electrode rods, said voltage being wherein U is the magnitude of a direct-voltage component, V is the amplitude of an alternating-voltage component and cos wt is cos 21rft, f being the frequency and 2 being the time.

5. A mass filter according to claim 1, wherein said collector electrode means comprising a separate ion collector for each of said analyzer electrode rods.

6. A mass filter according to claim 1, wherein said analyzer electrode rods have respectively different diameters.

7. A mass filter according to claim 1, wherein said analyzer electrode rods are positioned at respectively different radial distances from said axis.

8. A mass filter according to claim 7 for separating different masses, wherein said radial distances of said analyzer electrode rods from said axis are equal to c-mwherein m is the mass to be separated by an analyzer electrode rod and c is a constant.

9. An electric mass filter according to claim 1 wherein said analyzer electrode rods have different diameters and are positioned at respectively diifer'ent radial distances from said axis, said voltage supply means providing different voltage magnitudes to said dilferent analyzer electrode rods, each individual voltage being where U is the magnitude of a direct-voltage component, V is the amplitude of an alternating-voltage component and cos wt is cos 21rft, being the frequency and t being the time.

References Cited by the Examiner UNITED STATES PATENTS 2,939,952 6/1960 Paul et a1 2504l.9 3,129,327 4/1964 Brubaker 2504l.9 3,147,445 9/1964 Wuerker et al. 330-47 3,197,633 7/1965 von Zahn 2504l.9

RALPH G. NILSON, Primary Examiner.

W. F. LINDQUIST, Assistant Examiner. 

1. AN ELECTRIC MASS FILTER, COMPRISING AN EVACUABLE ENVELOPE, AN ION SOURCE, COLLECTOR ELECTRODE MEANS SPACED FROM AND DEFINING WITH SAID ION SOURCE AN ION PATH AXIS IN SAID ENVELOPE, PAIR OF ELONGATED ANALYZER ELECTRODE RODS EXTENDING BETWEEN SAID SOURCE AND SAID COLLECTOR ELECTRODE MEANS IN AXIALLY SPACED RELATION TO BOTH AND RADIALLY SPACED FROM SAID AXIS FOR APPLYING AN ALTERNATING ELECTRIC FIELD FOR LATERAL DEFLECTION OF IONS, SAID ANALYZER ELECTRODE RODS BEING SYMMETRICALLY POSITIONED TO AT LEAST TWO SYMMETRY PLANES THROUGH SAID AXIS, INTERMEDIATE SHEET ELECTRODES INTERMEDIATE TWO OF SAID SYMMETRY PLANES, SAID SHEET ELECTRODES HAVING A SUBSTANTIALLY UNIFORM THICKNESS SMALLER THAN THAT OF SAID ANALYZER ELECTRODE RODS AND EXTENDING RADIALLY THROUGH AND FROM SAID AXIS AND BETWEEN SAID ANALYZER ELECTRODE RODS, SAID ION SOURCE BEING MOUNTED ON SAID AXIS, SAID SHEET ELECTRODES BEING AT LEAST AS LONG AS SAID RODS IN AXIAL LENGTH, AND VOLTAGE SUPPLY MEANS CONNECTED TO SAID ANALYZER ELECTRODE RODS FOR SUPPLYING DETERMINED VOLTAGES TO SAID ANALYZER ELECTRODE RODS. 